The present invention relates to a display system, and in particular, to a method of operating such a display system.
It is known to provide a display system having a plurality of scanning lines and a plurality of signal lines arranged so as to form a matrix. The scanning lines extend generally perpendicularly to the signal lines in the array, and at each intersection of a signal line and a scanning line, there is a display unit. Each display unit is connected to the immediately adjacent scanning and signal line, and may be addressed by applying suitable signals to those scanning and signal lines. It can thus be seen that by sequentially applying such signals to the signal lines, and then, while each signal is applied to a signal line, sequentially applying signals to the scanning lines, each display element can be uniquely and sequentially addressed.
For a display system, each display unit may comprise a display element such as e.g. a liquid crystal display element, with the display element being connected via a switching element to the immediately adjacent switching line. The switching element may, for example, be a thin film transistor (referred to hereinafter as a "TFT"), and the gate electrode of that TFT is connected to the adjacent scanning line. Thus, when signals are applied to the scanning and signal lines connected to a display unit, the TFT is made conductive so that the display element may be actuated.
One problem with such a system is that if there is a short across the switching element, then there is a direct connection between the corresponding signal and scanning lines, which could cause malfunction of the device. To prevent this, it has been proposed in JP-A-61-250969 (1986) to provide a current limiting means in the form of a resistive element, between the source electrode of the TFT (i.e. the drive electrode which is not connected to the display element), and the adjacent signal line. The current limiting means prevents interference between the scanning voltage and the signal voltage, but the provision of such a current limiting means presents further problems.
Firstly, the resistance of the current limiting means has the effect of lengthening the time needed for writing a signal applied to the signal line to the display element. When the display element is a liquid crystal, and that liquid crystal is made large in area, the capacitants of the liquid crystal layer increases, thereby reducing the effective scanning time. When this is combined with the lengthening in the time for writing from the signal line to the display unit, the voltage written to the liquid crystal display element is insufficient, so that the contrast of the liquid crystal display element of the display unit is not sufficiently high. Furthermore, any variation in the resistance of the current limiting means will cause a variation in this effect, so that the brightness of the display unit may vary across the matrix of the display system.
Therefore, it has also been proposed, in JP-A-60-097384 (1985), to provide a current limiting means between the control electrode (gate) of the TFT and the scanning line.